High-luminous-efficiency and high-stability quantum-dot-based micro-nano-packaged fluorescent powder is used in quantum dot optimized white LEDs generally for regulating the color rendering index of the white LEDs, and is used in tricolor (red/green/blue) LED devices for displaying so as to improve the color gamut of a display as well.
Colloidal quantum dot has the advantages of continuously adjustable light emitting, high purity of color of emitted light and higher conversion efficiency, and has a relatively mature preparation technology as a core material for next generation lighting and displaying technology. In order to reduce or eliminate surface defects (e.g. intrinsic crystal growth defects and dangling bonds) of the quantum dots, a method commonly adopted is to epitaxially grow an inorganic shell on the surface of the quantum dot. However, as a nanocrystal particle having an inorganic shell structure, it still has very high surface energy, and changing the circumstance of the quantum dot may possibly cause clustering and fluorescence quenching of the quantum dots. In order to increase the self-stability of the quantum dots and the stability thereof in a composite material, a surfactant is usually used in organic modification on the surface of the quantum dots to obtain colloidal quantum dots.
In general, the quantum dots cannot be directly used because the quantum dots are relatively unstable compared to bulk materials. The quantum dots themselves may cluster to form larger particles. In addition, the colloidal layer may be eroded easily, and thus a defect level is left, forming a non-radiative channel. Most of the physical and chemical circumstances will cause fluorescence quenching to the quantum dots. Therefore, how to passivate quantum dot is a relatively hot and critical issue at present.
In practice, a common method is to directly disperse the quantum dots into a polymer matrix to obtain a fluorescent composite material, forming a simple structure for “quantum dot carrier material”. The composite material substantially maintains the fluorescence characteristics from the colloidal quantum dots. After that, the composite material is directly applied in a blue LED to obtain white light, or a multicolor spectrum converted by the quantum dot, respectively for direct use in displaying and lighting.
Some conventional methods for packaging quantum dots in present literatures include the followings. 1. Dispersing the quantum dots into polymethyl methacrylate (PMMA), wherein remote packaging (see reference 1) is used. This packaging form has the defects that the quantum dots will aggregate in the PMMA slowly, leading to fluorescence radiation wavelength red shift and fluorescence efficiency drop as the quantum dots are not compatible with surface ligands and barrier layer. Secondly, as the water-resistance and oxygen-resistance abilities of the PMMA are poor, the osmotic water-oxygen micromolecules may easily erode the surfaces of the quantum dots, leading to fluorescence drop. 2. Coating on chip (On chip) (see reference 2) packaging. In order to prevent the quantum dots from aggregation, a transamination treatment is conducted on the surfaces of the quantum dots firstly, to enhance the compatibility between the quantum dots and the barrier layer material around the quantum dots, thereby to decrease aggregation and prevent water and oxygen erosion. However, this transamination itself may easily destroy the surface ligands of the quantum dots, thus affecting the initial fluorescence efficiency of the quantum dots.
Furthermore, the quantum dots are relatively sensitive to temperature, oxygen and water. Even in a composite material, the quantum dots may fallen off, leading to fluorescence drop, due to long-term oxidation of the surface ligands of the colloidal quantum dots and incompatible problem between the quantum dots and the polymer carrier material, this severely affects the service life of a device. In order to improve the service life of the quantum dot device, improvement is made based on the remote packaging. A silica (silica)/polyvinylpyrrolidone (PVP) material has been used as a barrier layer in remote packaging (see reference 3). A composite film with surface protection is introduced, specifically a film having better water-resistance and oxygen-resistance ability is added between an upper layer and a lower layer for preventing water and oxygen invasion, thereby to reduce damages to the surfaces of the quantum dots. Although with this method the water and oxygen may be obstructed to a certain extent, ligand loss and aggregation effect caused by compatible problem between the outer surfaces of the quantum dots and a carrier cannot be avoided, thus the stability and reliability of a light emitting device are impaired.
For a method of directly compositing the quantum dots with silica gel, the colloidal ligand at the outer layer of the quantum dots is not compatible with the silica gel. In particular, some surface ligands containing sulfur (S) will be effected with platinum (Pt) in the silica gel, and thus the solidification of the silica gel will be affected, the silica gel may not solidify. In addition, a non-solidified silica gel will usually cause clustering due to the compatibility problem between the surface ligands and the silica gel.
For a quantum dot polymer composite material, the fluorescence characteristics of the quantum dot polymer material obtained via polymerization in a quantum dot dispersion liquid are usually affected by an initiator, a polymer active site or the like, or fluorescence drop or quenching may occur as affected by a high polymer chemistry molecule polymerization reaction. Similarly, for a physical blending composite material, which is obtained through directly dissolving a high polymer using a solvent, then dispersing the quantum dots, and volatilizing the solvent, clustering of the quantum dots is caused due to the compatibility problem between the quantum dots and the polymer. On the other hand, due to osmosis, the micromolecules, for example, water molecules and oxygen, usually interacts with the quantum dots, eroding the surfaces of the quantum dots, thereby leading to light emitting defects and fluorescence efficiency recession.
Direct processing, such as silica growing and surface transamination modification, is conducted on the surfaces of the quantum dots. The method presently has a major problem that an apparent fluorescence quenching effect (see reference 4) may occur for the quantum dots due to the substitution of the surface ligands. Therefore, for the use of the high-luminous-efficiency and high-stability quantum dots in a device, the following problems have to be solved: 1. the quantum efficiency of the quantum dots cannot be impaired during the surface treatment of the quantum dots or composition with other materials; 2. the carrier circumstance of the quantum dots shall be compatible with the surfaces of the quantum dots, this prevents the quantum dots from self-aggregation on one hand, and prevents the ligands from falling off on the other hand; and 3. a barrier layer is set to prevent micromolecules like water vapor and oxygen from eroding the surfaces of the quantum dots.